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  1. If
    x24 + 1
    		 = 7 , then the value of
    x72 + 1
    		 is
    x12x36

    1. 433
    2. 322
    3. 343
    4. 432
Correct Option: B

x24 + 1
 = 7
x12

⇒
x24
 +
1
x12x12

⇒ x12 +
1
 = 7
x12

∴
x72 + 1
 =
x72
 +
1
x36x36x36

= x36 +
1
x36

= x12 +
1
3 - 3 . x12 .
1
x12 +
1
x12x12x12

[ ∴ a3 + b3 = (a + b)3 - 3ab(a + b)]
⇒ x36 +
1
 = 73 - 3 × 7 = 343 – 21 = 322
x36


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